1. Technical Field
The present invention relates in general to hard disk drives and, in particular, to an improved system, method and apparatus for strain-assisted magnetic recording that allows controlling magnetic switching field value and tightening of magnetic switching field distribution in bit patterned media.
2. Description of the Related Art
Bit patterned media (BPM) is a leading candidate to extend the data areal densities of magnetic recording beyond those achievable by conventional continuous perpendicular magnetic recording based on granular recording media. The islands of BPM need to be sufficiently small and of sufficient magnetic quality to support high bit areal densities (e.g., at 1 Tb/in2). The islands have diameters that are approximately 15 to 20 nm (e.g., assuming a unit cell of 25.4 nm2), with trenches having widths of about 10.4 to 15.4 nm, and a bit aspect ratio (BAR) of about 1. Moreover the switching field distribution (SFD) may need to be smaller than 1000-1500 Oe, depending on the head field gradient and other system parameters. See, e.g., M. E. Schabes, “Micromagnetic Simulations for Terabit/in2 Head/Media Systems,” J. Magn. Mag. Mat., (2008). Furthermore since the field of the write head becomes smaller as the size of the write head is decreased, maintaining the writeability and thermal stability of the islands are issues for BPM.
One critical issue for the development of BPM is that the SFD (i.e., the bit-to-bit variation of the coercive field) should be narrow enough to secure exact addressability of individual predefined bits without overwriting adjacent bits. The SFD has many origins, such as variations in the patterned dot sizes, shape and spacing, intrinsic magnetic anisotropy distribution, and dipolar interaction.
Another critical issue for the increase of BPM areal density is how to reduce the lateral size of the bit-dots, as well as the distance between two neighbors without decreasing the thermal stability of the magnetic media. To do so, a large increase in the media anisotropy is required, although the magnetic write field cannot be increased extensively due to head field limitations. Energy-assisted recording has been proposed to locally decrease the switching field value during the writing process while keeping a very good thermal stability at rest.
To solve the SFD problem and to control the switching field value while keeping a good thermal stability in the media, one aspect of the present invention is to change the anisotropy amplitude or the anisotropy direction of the magnetic media before and/or during the write process.
Change in the amplitude and direction of the anisotropy can be achieved by adding one or more transition layers that induce a strain on the media layer via its own structural changes. See, e.g., H. Boukari, et al., J. Appl. Phys. 101, 054903 (2007); and J. W. Lee, et al., Appl. Phys. Lett. 82, 2458 (2003).
For a magnetic layer whose anisotropy is locally controlled via a strain induced by a strain-inducing layer (SIL), the strain transferred from the SIL to the magnetic layer (ML) is increased as the lateral dimension of the magnetic part is decreased. In the case of continuous granular media currently used in hard disk drives, the strain affecting one bit is constrained by the neighboring bits. In bit patterned media technology, each bit consists of a single island that is physically separated form the neighboring bits. As a consequence, there is no lateral brake to the SIL and ML volume variations of the bit and the effect of the strain on the ML magnetic anisotropy is maximal.